The present invention relates generally to ceramic honeycomb structures, and in particular to honeycomb structures such as honeycomb flow-through catalyst substrates and wall-flow particulate filters.
Soot particles are removed from diesel exhaust typically using a wall-flow honeycomb filter. FIG. 1 shows a prior-art ceramic honeycomb structure in the form of a wall flow filter 100. An array of parallel, straight cells 104 adapted for fluid flow runs axially along the length of the honeycomb filter 100. The cross-section of the cells 104 is typically square. The cells 104 are defined by an array of interconnecting porous webs 106 running along the length of the honeycomb filter 100 and intersecting with the skin 105 of the honeycomb filter 100. The cells 104 are end-plugged with filler material 107 in a checkerboard pattern at the end faces 108, 110 of the honeycomb filter 100. Diesel exhaust 112 enters the honeycomb filter 100 through the unplugged ends of the cells 104 at the end face 108, flows from one cell 104 to another through the porous webs 106, and emerges through the unplugged ends of the cells 104 at the end face 110, with the porous webs 106 retaining a portion of the soot particles. The efficiency of the honeycomb filter 100 is directly proportional to the amount of soot particles retained by the porous webs 106 with each pass of the exhaust.
As the soot particles accumulate on the porous webs, the effective flow area of the honeycomb filter decreases. This decreased effective flow area creates a pressure drop across the honeycomb filter, which leads to a gradual rise in back pressure against the diesel engine. When the pressure drop becomes unacceptable, thermal regeneration is used to remove the soot particles trapped in the honeycomb filter. During thermal regeneration, excessive temperature spikes can occur, producing thermal stress in the honeycomb filter 100. If the thermal stress exceeds the mechanical strength of the honeycomb filter 100, the honeycomb filter 100 can crack. This is particularly a concern where the honeycomb filter 100 is made of a low tensile strength material, such as ceramic.
Ceramic honeycomb flow-through catalyst substrates have the general honeycomb structure of filter 100, but do not incorporate end plugs of filler material 107 in the channels of the honeycombs. Thus they operate to treat engine exhaust gases as the exhaust flows directly through the channels of the substrates. While such flow-through substrates do not require regeneration to remove trapped particulates, they are nevertheless subjected to substantial stresses during catalytic reactor assembly and in the course of use. Further, advanced catalyst substrates typically incorporate porous webs 106 of very slight thickness, e.g. thicknesses in the 25-150 μm range, rendering them less mechanically durable than thicker-walled substrates.
Providing honeycomb substrates and filters with sufficient mechanical strength to withstand thermal shocks and filter regeneration without decreasing the performance of the honeycomb substrates and filters is challenging because mechanical strength and pressure drop tend to be inversely coupled. For example, for a honeycomb filter having a given effective flow area and pressure drop, the general approach to improving the mechanical strength of the honeycomb filter has been to thicken the porous webs. This modification has the advantage of increasing the thermal mass of the honeycomb filter but it creates a different problem in that web thickening reduces the effective flow area of the honeycomb filter. This reduction in the effective flow area of the honeycomb filter results in increase in pressure drop across the honeycomb filter, even before soot particles accumulate on the porous webs. To minimize the increase in pressure drop across the honeycomb filter, the porosity of the webs may be increased such that the effective flow area of the modified honeycomb filter is close to the given effective flow area. However, increasing the porosity of the webs is problematic in that it produces a corresponding decrease in the thermal mass of the honeycomb filter.
From the foregoing, there is an ongoing desire to improve the resistance of ceramic honeycombs such as honeycomb filters and catalyst substrates to cracking failure under mechanical loads and/or thermal regeneration that does not problematically increase pressure drop across the honeycomb structures in use.